The expression “mobile receiver” will be understood here to mean both the devices (or receivers) exclusively dedicated to satellite positioning, portable or embedded in a land, sea or air vehicle, and the communication terminals equipped with a satellite positioning device, such as, for example, cell phones, or laptop computers or personal digital assistants (PDA), possibly of communicating type.
In a satellite positioning system using GNSS (Global Navigation Satellite System) type receivers, the data signals enabling the receiver to compute its positioning originate from different satellites belonging to a constellation of positioning satellites (at least four to determine four unknowns corresponding to the geographic coordinates x, y, z and time coordinate t of the receiver).
Satellite positioning involves a sequencing of two steps. The first step, called acquisition, consists in determining, on the mobile receiver concerned, the pseudo-random spreading codes which modulate the signals originating from the satellites belonging to the constellation and related to a reference time. The procedure in fact “compares” the signals received from the satellites to replicas of signals generated locally by the receiver and resulting from assumptions concerning the reference time and concerning the pacing frequency of the satellites, in order to deduce therefrom the pseudo-random codes which modulate said received signals or, in other words, to synchronize the pacing clock of the receiver and its frequency on the clock and the frequency of each satellite. For this, a search in time-frequency for the energy of the signal originating from the satellite is performed, this search usually being carried out by correlation measurements based on pairs of time and frequency assumptions in order to determine the maximum correlation between the received signal and the local replica of the receiver.
The second step consists in determining the position of the mobile receiver on the basis of the acquired codes and navigation data notably contained in the received signals. This second step may be more specifically subdivided into three substeps: a substep for determining, from the acquired pseudo-random codes, the propagation times of the signals between each of the satellites and the receiver, a substep for determining, from the navigation data contained in the signals and the propagation times, pseudo-distances between the receiver and each of the satellites, and a substep for determining the position of the receiver from the pseudo-distances. An exemplary satellite positioning system is described in the document US 2006/0115022.
The accuracy of each propagation time, and therefore of each pseudo-distance, directly determines the accuracy of the position. Now, the accuracy of each propagation time depends on the quality of the acquisition of the pseudo-random codes of the corresponding received signal, which is dependent on the quality of said received signal. Consequently, when at least one of the signals received from a satellite is of poor quality, which is relatively commonplace, notably in uneven or crowded environments such as urban areas, the determined position is usually affected by error. It is even possible to be momentarily unable to determine the position of the receiver, even though the signals originating from the other satellites are of good quality.
The receiver has three frequency uncertainties to which is added an unknown concerning the date leading it to perform the search in time and in frequency for the energy originating from a satellite. These three frequency uncertainties are the Doppler effect associated with the mobility of the satellite, the uncertainty linked to the accuracy of the clock of the receiver, and the Doppler effect associated with the mobility of the receiver. The Doppler effect associated with the movement of the satellite can be determined in an known manner by using, for example, an assistance server such as that used in the AGPS (Assisted GPS)-type locating techniques. The local oscillators of the clocks are increasingly efficient and becoming more and more stable. The Doppler effect associated with the movement of the receiver becomes the predominant source of the uncertainties concerning the location of the correlation peak and therefore of the receiver.
Not taking into account the Doppler effect associated with the receiver may have a dramatic effect in the case of the acquisition of low energy signals which require the received signal to be integrated coherently over a long time period. In practice, the width of the frequency assumption integration windows being inversely proportional to the integration time, the longer the coherent integration time, the smaller the width of the frequency assumption integration windows and therefore the greater the number of frequency assumptions. The time and frequency sweeps then involve a computation power and data processing time that are very significant for the receiver and increase the uncertainty concerning the location of the receiver. In a conventional acquisition scheme, as described, for example, in the document US 2006/0012515, the coherent integration is performed generally on a frequency assumption which remains the same throughout the integration time, which presupposes that the actual frequency of the received signal is stable over the integration time. During a long coherent integration, the Doppler associated with the movement of the user causes the actual received frequency to vary during said integration which makes the integration inoperative if it is performed on a stability assumption.